The field of the invention is recovery of gases from the offgas in a pressure swing adsorption unit in hydrogen production.
Pressure swing adsorption (PSA) is a well known process for re cove ring light gases from mixtures which also contain heavier, more readily adsorbable components, and the recovery of hydrogen from steam-methane syngas containing hydrogen, carbon oxides, and methane is a particularly well suited application of the PSA process.
A typical PSA process and apparatus is described in U.S. Pat. No. 3,430,418 to Wagner and in U.S. Pat. No. 3,986,849 to Fuderer, both of which are incorporated by reference herein. With an increasing demand of highly purified gases, various improvements were developed to help overcome limitations in the original process with respect to flow rates, capacity, and yield. For example, Fuderer describes in U.S. Pat. No. 4,333,744 an increase in yield employing a particular PSA loading pattern in which a first portion of a feed stream is pretreated to remove substantial amounts of an unwanted component, and a second portion is loaded in an untreated form. Although the Fuderer""s methods may increase the overall amount of purified gas and may advantageously increase the flow rate, the offgas is eventually directed to a waste line, or into a reformer burner, and components of interest in the offgas are concomitantly lost.
To avoid concomitant losses of desirable components in the offgas stream, various recovery methods were developed. One method of recovering desirable components employs membranes that either concentrate or filtrate hydrogen in the offgas stream. For example, G. Intille describes in U.S. Pat No. 4,229,188 the use of hydrogen-permeable membranes to recover H2 from the offgas. Intille""s membranes advantageously remove H2 with high selectivity in a single process step, however, the use of such membranes typically requires relatively high pressure, thereby increasing the overall energy demand. To avoid at least some of the problems associated with hydrogen-permeable membranes, Anand et al. teach in U.S. Pat. No. 5,435,836 the use of an adsorbent membrane. Adsorbent membranes generally allow hydrogen recovery at comparably low pressure with relatively high specificity. The advantage of relatively low pressure, however, tends to be offset by the need of membrane exchange, thereby either increasing the complexity of the hydrogen plant. or necessitating discontinuous operation.
Another method utilizes a serial configuration of PSA units, wherein a first PSA unit has a different selectivity from a second PSA unit, and wherein the offgas from the first unit is directed to the feed end of the second PSA unit. An example for this configuration is described by R. Kumar in U.S. Pat. No. 4,913,709. Kumar""s serial configuration of PSA units with beds having non-identical adsorption specificity is favorable because relatively high volumes of offgas may be purified at a time. However, the complexity and number of coordinated cycle steps generally increases due to the different physico-chemical properties of the adsorbent beds.
In still another method, U.S. Pat. No. 4,553,981 to Fuderer, the feed gas of a PSA unit is pretreated to remove a second component at least in part, while the PSA unit purifies a first component. A first portion of the offgas of the PSA unit is recycled into the same PSA unit, and a second portion of the offgas is directed to a waste line. Alternatively, a first portion of the offgas is directed to a second PSA unit having the same specificity, and the offgas of the second PSA unit is fed to a waste line. By removing the second component from the feed gas before the feed gas enters the PSA unit, the offgas will typically have a higher relative purity, and a remaining portion of the first component may therefore be easier to extract. However, the second component in Fuderer""s configuration typically needs to be further purified.
Although various improvements have been developed to increase the recovery rate of desirable components in the offgas from PSA units, all or almost all of them have one or more than one disadvantage. Thus, there is a need to provide methods and apparatus for increased recovery of desirable components in the offgas from a PSA unit.
The present invention is directed to a gas separation apparatus that has a first pressure swing adsorption (PSA) unit receiving a feed gas comprising a first and a second component. The first PSA unit produces a first product gas predominantly comprising the first component, and a first offgas comprising at least some of the first component and the second component. A compressor is coupled to the first PSA unit and compresses the offgas to form a compressed offgas, and an absorber unit downstream of the compressor employs a solvent to remove at least part of the second component from the compressed offgas, thereby forming an enriched compressed offgas. A second PSA unit receives the enriched compressed offgas and produces a second product gas predominantly comprising the first component, and a second offgas. The gas separation apparatus may further comprise a flash unit and a gas liquefaction unit, which is preferably an autorefrigeration system.
In one aspect of the inventive subject matter, the feed gas is an effluent gas stream from a steam reformer and/or shift converter, and preferably comprises H2 and CO2 in excess over CO, CH4, and other gaseous products. The first and second PSA units are preferably hydrogen PSA units, and while the offgas from the first PSA unit is used to recover first and second components, the offgas from the second PSA unit is preferably routed to a reformer burner.
In another aspect of the inventive subject matter, a method of recovering a first and a second component from an offgas of a PSA unit includes a first step in which the offgas is compressed to produce a compressed offgas stream. In a next step at least some of the second component is recovered from the compressed offgas stream to produce an enriched compressed offgas stream. The enriched compressed offgas stream is passed to a second PSA unit to recover at least some of the first component.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components.